We have isolated and characterized a number of muscle-specific and "house-keeping" genes that are differentially expressed during myogenesis (muscle formation) in vitro. These include the alpha cardiac (embryonic muscle and cardiac specific isoform), alpha skeletal (adult skeletal muscle isoform) and beta cytoplasmic actin genes, and the myosin light chain 1f/3f gene. The alpha actin isoforms and the myosin light gene are only expressed in muscle whereas the beta actin genre is expressed in practically all undifferentiated cells and is transcriptionally suppressed in muscle. We have defined the cis regulatory regions of these genes responsible for this regulation in muscle and have made preliminary attempts to study the transacting proteins that interact with these regions. Recently we have isolated two different avian genes (CMD1 and CMGN1) that are involved in myogenic determination since the expression of either one of these genes in fibroblasts coverts the cells to a stable muscle phenotype. Cotransfections with either of these genes and a muscle-specific promoter, such as the alpha cardiac actin promoter, joined to a reporter gene like CAT (chloroamphenicol acetyltransferase) or LUC (luciferase) activates the reporter expression. We now have in hand two specific muscle factors and a series of differentially regulated genes from muscle and we are studying the molecular basis of this factor regulation of muscle gene expression/suppression.